Why do some games force you to unlock content before you can use it?

fraggle said:

If there's some goal you're working towards (unlocking weapon X) then you have some incentive to keep playing, and dangling carrot of curiosity.

I'm more amazed that there are people who need superficial goals like achievements or unlocks to keep playing rather than playing for the sake of beating the game or for just having fun. :/

Perhaps games just aren't as good as they used to be anymore?